mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Horizontal Gene Transfer of Epigenetic Machinery in Apicomplexan Parasites
Organisms in the phylum Apicomplexa are a group of about 5000 protists possessing several unique characteristics. They are all obligate, intracellular parasites in animals and have a unique organelle, called the apicoplast. The apicoplast is a product of secondary endosymbiosis, whereby a free-living protist engulfed a single celled red algae. Over time, the nucleus and photosynthetic ability was lost, however, the apicoplast is still an essential organelle to the parasite. The group of parasites infects a wide range of hosts and contain several significant human pathogens. Among these are the causative agent of malaria, Plasmodium spp., Toxoplasma gondii and Cryptosporidium spp. From an evolutionary perspective, they all evolved from a free-living or symbiotic ancestor but lost and aquired genes which made them more suitable to life as an obligate parasite. This is a fairly dramatic lifestyle shift which would require the acquisition of cytoadhesion molecules, immune evasion tactics and a shift in metabolic machinery to one more suitable for intracellular living as opposed to the environment. 1,2 Apicomplexan Acquisition of Gene Regulation Elements Not only did the environment for the life of an apicomplexan shift dramatically, apicomplexan parasites generally have a fairly complex life cycle. Their life cycle usually involves multiple stages in potentially several different host species. This type of adjustment requires gene expression coordination and regulation. Many of the factors required for gene regulation have been acquired by horizontal gene transfer or de novo ''innovation. Horizontal gene acquisition has occurred through transfer from the algal endosymbiont and the host species. The ApiA2 group of transcription factors was acquired from the algal endosymbiont to control gene expression and factors involved in O-linked glycosylation and cytoadhesion have been acquired from the host. In a 2013 study published in BMC Evolutionary Biology, Kishore ''et. al. discovered a new set of genes acquired through horizontal gene transfer. The set of genes is thought to be involved in histone lysine modifications which are key epigenetic elements in gene expression. These could control genes involved in immune evasion and host cell invasion. In the study they analyze the acquisition of SET domain containing proteins, including Set8 and Set2, which are involved in histone lysine methyltransferase activity and regulation. Using phylogenetic analysis, the authors found apicomplexan Set8 to most closely resemble animal Set8 proteins. Other organisms, in particular the dinoflagellates, which are more closely related to apicomplexans did not show any significant hits for the Set8 protein. This infers that the gene was acquired through an independent horizontal gene transfer. The authors also theorize that the acquistion came from a nematode and happened prior to radiation in the apicomplexan phylum, but post separation of apicomplexa and dinoflagellates and colopodellids. In contrast, when the authors analyzed Set2 and JmjC1 which work as a methylase and demethylase respectively on H3K36, they found Set2 to be more closely aligned with Ashr3 an H3K36 methyltransferase found in green plants. Given that the Ashr3 Set family is not found in animals or fungi, it is possible the gene was acquired from the algal endosymbiont. In contrast to both the Set genes, no evidence for horizontal gene transfer was obtained for JmjC1 illustrating within just these three genes the many ways apicomplexan parasites have evolved over time. 3 References 1. http://en.wikipedia.org/wiki/Apicomplexa 2. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html 3. Kishore, S.P. et.al. Horizontal gene transfer of epigenetic machinery and evolution of parasitism in the malaria parasite Plasmodium falciparum and other apicomplexans. BMC Evol Biol. 2013 Feb 11;13:37. doi: 10.1186/1471-2148-13-37. http://www.biomedcentral.com/1471-2148/13/37